Within a packaged integrated circuit (IC), the power and ground typically have a capacitance between them due to added small capacitors, and the sum of various transistor features. This capacitance can exceed several hundred pico farads (pF), depending on the chip size and design. In the typical case, the silicon is bonded out to the package pins with thin gold bonding wires that are usually about 1 mm in length. These bonding wires will have an inductance of about 1 nH/mm. So a typical IC will have the chip ground separated from the outside ground by about 1 nH, and the chip power separated from the external power (e.g., VCC) by about 1 nH. Outside of the circuit, the power and ground are typically connected together through a large decoupling capacitor (e.g., 0.1 uF). The trace going from the chip VCC pin to the decoupling capacitor, the decoupling capacitor itself, and the trace from the other end of the decoupling capacitor to the chip ground constitute another parasitic inductance between the VCC and GND pin external to the package. Together with 200 pF of internal chip capacitance from VCC to GND, a resonant tank is formed, with a resonant frequency of about 250 MHz in this example. Often, resonances from about 100 MHz to 200 MHz are seen, depending on the size of the loop capacitance and inductance.
The above described resonance causes problems within high frequency chips. For example, when the power supply current is varied by the circuit activity, the chip power will ring at the resonant frequency as a result. This will cause the chip ground to ring, as well as the chip power, and can be a source of electromagnetic interference (EMI) problems. In addition, the ringing can pollute the signals within the chip, and in severe cases cause the chip to oscillate or misbehave in some other manner.
A potential solution is to use resistors in series with internal decoupling capacitors to try to damp out this problem. While this helps, it is not always an acceptable solution. Additionally, there are some capacitors that may not lend themselves to having a series resistor, and the series resistor destroys the ability of the capacitor to decouple above the RC frequency of the solution. Thus, if the resistor is chosen to limit resonance at 150 MHz, above 150 MHz the capacitor looks like a resistor, instead of providing ever more protection against supply variation.
Accordingly, there is a need to overcome the above discussed problems of the insufficiently damped parasitic resonant circuit.